The present invention relates to a random access method in a long term evolution (LTE) based mobile communication system for supporting a random access having highest compatibility with conventional LTE with a cell size of 100 kilometers (km) or more and with a power limited terminal, and a preamble structure for the random access.
In the future, a mobile communication network is expected to develop for combination or cooperation between a terrestrial network and a satellite network. In the integrated satellite and terrestrial system, community between a satellite and a terrestrial wireless interface is an essential matter in consideration of cost for a terminal. In particular, considering that an LTE-based terrestrial mobile system is considered as a next generation international mobile telecommunication (IMT)-advanced system, there is a great demand for research on an LTE-based satellite wireless interface which has an even larger cell size and longer round-trip delay (RTD) compared to those of a terrestrial network, and considers a power limited satellite network environment.
To help cell search which is a process for synchronization with a cell in a network, two particular signals such as a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) are transmitted to an LTE downlink. Two PSS of one frame are identical in one cell. The PSS in one cell may have three different values according to a cell identifier (ID) of a physical layer of the cell. More specifically, the three cell IDs in one cell ID group may correspond to different PSS, respectively.
In any cellular system, random access of a terminal is basically required to request a connection setup with a network. In the LTE, the random access is used for a few purposes as follows.                To form a wireless link as an initial connection (move from Radio Resource Control(RRC)_IDLE to RRC— CONNECTED)        To form a wireless link again after the wireless link fails        To establish uplink synchronization with a new cell as a handover        To establish uplink synchronization when uplink or downlink data arrives in a state in which a terminal is in an RRC_CONNECTED state but uplink synchronization is not established        To request for scheduling when scheduling request resources designated on a physical uplink control communication (PUCCH) are absent        
In all cases above, a main purpose of the random access is establishment of uplink synchronization at an initial connection. Also, the random access is performed to allocate Cell Radio Network Temporary identifier (C-RNTI) which is an only identifier of the terminal.
A main purpose of transmitting a preamble is in informing a base station that random access has been tried and helping the base station estimate a delay between the terminal and the base station. The delay is estimated to adjust uplink timing. Time-frequency resources through which the random access preamble is transmitted are called a physical random access channel (PRACH). A network broadcasts time-frequency resources available for transmission of the random access preamble to all terminals. During the random access, the terminal selects one preamble to be transmitted through the PRACH.
A length of a preamble region in a time domain is determined by preamble setup. A length of random access resources is basically 1 ms. However, a longer preamble may be set. Theoretically, an uplink scheduler of evolved node B (eNodeB) may remain a long random access region by avoiding scheduling of terminals in a plurality of consecutive subframes.
FIG. 1 illustrates a process of random access of 3rd Generation Partnership Project (3GPP) LTE selected as an international mobile telecommunication (IMT)-Advanced wireless interface technology. Referring to FIG. 1, the random access process of LTE may include four steps. In a first step, synchronization is obtained through PSS/SSS from a base station, and system information is obtained through a broadcasting channel (BCH). The information transmitted through the BCH may include parameters for generating a random access preamble. In a second step, the terminal transmits the random access preamble. Therefore, the eNodeB may estimate a transmission timing of the terminal. Uplink timing estimation is an indispensable process in orthogonal frequency division multiplexing (OFDM)-based LTE. Without setting synchronization, uplink data cannot be transmitted. In a third step, a parameter of the transmitted preamble is extracted from the base station. In a fourth step, the terminal having the parameter of the preamble detected from the base station transmits a preamble ID, an access permission message, and Timing_Advanced information necessary for the uplink synchronization, to inform that the uplink synchronization is obtained. Last, the terminal adjusts the Timing_Advanced information based on the signal transmitted from the base station and request resources of the base station.
FIG. 2 illustrates a random access preamble format. Referring to FIG. 2, the random access preamble currently defined by the random access process includes a cyclic prefix (CP), a preamble sequence, and a guard time (GT), and has four formats. Format 0 includes lengths of the CP and the GT of 0.1 ms, respectively. Format 0 supports a cell size of about 15 km. Format 1, Format 2, and Format 3 support a cell size of about 78 km, 30 km, and 100 km, respectively. Formats 2 and 3 transmit the preamble sequence twice to increase an energy gain. For transmission of the preamble, the GT is used to cope with uncertainty of timing. Before the random access begins, the terminal obtains downlink synchronization from a cell search process. However, before the uplink synchronization is established, location of the terminal in the cell is not yet known and therefore uplink timing is still uncertain. According to an increase in the cell size, the uncertainty of the uplink timing is increased. To take the uncertainty into consideration and avoid interference with following subframes unused for the random access, the GT may be used as part of the preamble transmission. For this purpose, the GP needs to be set larger than a sum of a RTD time difference between a nearest terminal and a farthest terminal with respect to the eNodeB and a multipath preamble delay time. In case of Format 4 having a longest GT, a cell size is 100 km or less. Therefore, in a mobile communication network having a cell size of 100 km or greater, such as a satellite mobile communication network, a conventional preamble format may not solve the uncertainty of uplink timing and the interference with the following subframes.
When the preamble includes the CP, it is efficient since the base station may process a low-complexity frequency domain. For this, a length of the CP needs to be greater than the RTD time difference between the nearest terminal and the farthest terminal with respect to the eNodeB, and may be almost equal to the length of the GT. Since any uplink transmission is not scheduled with respect to the subframes following the random access resources, a protection region larger than as shown in FIG. 2 may be generated. Therefore, when the cell size is about 100 km or more as in the satellite mobile communication network, frequency domain processing through one time of fast Fourier transform (FFT) window is impossible since the RTD time difference between terminals exceeds the length of the CP. Accordingly, time domain processing needs to be performed using a plurality of windows. As a result, complexity of a receiver and a preamble obtaining time is increased.
Finally, in the satellite mobile communication network, high output signal transmission is necessitated due to a small link margin. Considering a maximum transmission power level of a handheld terminal, which is relatively low, a transmission band width of 1 MHz or more of the uplink random access preamble in the LTE may not provide transmission power satisfying the link margin of the satellite system, by power allocated from respective subcarriers of a transmission block. Furthermore, since the lengths of GT and CP, which are considered to support a mobile communication network having a large cell area, need to be increased than in the conventional LTE, an entire preamble length needs to be increased on a time axis. Considering that the preamble bandwidth of the conventional LTE is 1 MHz or greater, the increase in the preamble length on the time axis may cause reduction in data transmission resources.